A frequency hopping communications system utilizes many frequency channels to send information from a transmitter to a receiver. The information is sent in the form of a series of codes having redundant information. Each code sequence or codeword represents one bit of information to be transmitted. For example, a codeword of “0100110001110000” may be used to transmit a logic state of one; whereas, a codeword of “1011001110001111” may be used to transmit a logic state of zero. In this example, sixteen bits are used to represent a single information bit such that the redundancy is fifteen bits.
In operation, a transmitter transmits the codeword rather than the bit itself. On the receiver side, codewords must be decoded by a decoder. To properly decode each codeword, the decoder must know which bit is the first bit of each codeword. In other words, the operation of the decoder must be synchronized with the incoming bit stream. One readily employed technique is to identify a known synchronization pattern of repeated codewords within the incoming bit stream.
While it is possible to achieve modem acquisition under very high bit error rate conditions (e.g., exceeding 20%) in a frequency hopping communication system, current decoder synchronization methods require synchronization information to have bit error rates in the range of 10% or less for reliable synchronization. However, synchronization should occur with a much higher error tolerance in the synchronization information. Therefore, it is desirable to provide a more robust technique for identifying the synchronization pattern in an incoming bit stream, whereby reliable synchronization can occur at bit error rates in excess of 20%.
While the following description is provided in the context of a frequency hopping communication system, it is readily understood that the synchronization schemes of this disclosure are applicable to other types of communication system which employ encoded or encrypted data streams. Additionally, the statements in this section merely provide background information related to the present disclosure and may not constitute prior art.